1. Field of the Invention
The present invention relates to a pressure device for pressure bonding an integrated circuit (IC) on a printed circuit board.
2. Description of the Related Art
Recently, in electrical circuit devices having IC chips mounted on circuit boards, advances have been achieved resulting in the increase in electrical signal velocity and integration density, and in the reduction in size. As a result, signal wiring is becoming fine in pitch and the number of pins is increasing. Along therewith, it has become more difficult to obtain positional accuracy and connection reliability in the process of bonding IC chips on a circuit board.
As a method for mounting an IC chip on a circuit board such as a ceramic substrate for a micropattern, silicon substrate, and glass substrate for liquid crystal, at present, wire bonding, a solder bump, stud bump, anisotropic conductive film (ACF), anisotropic conductive paste (ACP), nonconductive film (NCF), and nonconductive paste (NCP) are combined. Also, in such a mounting method, a chip on board (COB) device is used. For example, as such a COB device, a device adaptable to work with a wide space between ICs is known, in which an IC chip is bonded with a melted solder bump. Since the solder bump has a self-alignment mechanism itself, an allowable range in parallelism between the substrate and IC chips is large in such a COB device.
In the typical COB device, IC chips and the circuit board are sandwiched to perform heating and pressure bonding with a pair of tools arranged at the top and bottom. One such COB device is a one-position fixed type having one heating and bonding position with the tool. Another type of COB device is a plural-positions moving type, in which one of the tools arranged at the top and bottom (the top tool, for example) is fixed and the other (substrate-mounted stage) is moved. An example of the latter type COB device using the ACF is disclosed in Japanese Patent Laid-Open No. 2000-68633. In this disclosure, an elastic sheet is interposed therebetween, in order to reduce nonuniformity in pressure due to differences in height.
However, the following problems have been encountered in the conventional mounting method. Like reference characters will be used to designate like members in FIGS. 1 to 8.
A COB device may be at the one-position fixed type, as shown in FIG. 1, and the plural-positions moving type, shown in FIG. 2, in which a stage with a circuit board 5 is moved. In either type, in order to uniformly pressurize bumps 4 (not visible in FIG. 2) and an IC chip 3 in the direction of arrow 8, an upper tool 1 and a lower tool 2 are provided. A mechanism for adjusting parallelism between the tools 1 and 2 is provided in at least one of the tools (the upper tool in FIGS. 1 and 2). Referring to FIGS. 1 and 2, the upper tool 1 is slidable in directions of arrows 6 and 7. FIG. 3 shows a one-position fixed type in a state that the IC chip 3 is press-bonded after the parallelism of the upper tool 1 is adjusted relative to inclination of the press-bonding surface of the lower tool 2.
However, the parallelism adjusting mechanism mentioned above is for adjusting the parallelism between the press-bonding surfaces of the tools and not for adjusting the parallelism relative to each IC chip and circuit board. Therefore, as shown in FIG. 4, there is a first problem that for fluctuations in thickness of individual circuit boards 5 and IC chips 3 and fluctuations in height of individual bumps 4, uneven abutment and nonuniformity in a pressurization force are produced, and along therewith, nonuniformity in a connection resistance and instability in the connection reliability may be produced.
In the plural-positions moving type of the lower tool 2 (substrate-mounted stage), the upper tool 1 is provided with the parallelism adjusting mechanism, the upper tool 1 being parallel-adjusted at a predetermined position corresponding to every press-bonding position of the lower tool 2. Also, in this type, in addition to the same problem as in the one-position fixed type, errors are produced in a parallel-adjusted position due to fluctuations in positional accuracy of the reciprocating stage.
A second problem is that if the IC chips to be connected are arranged at a small pitch, it may occur that a number of the IC chips cannot be connected, because of the size and shape of the upper tool 1 for heating and pressurizing the IC chips.
A third problem is that when a plurality of the IC chips 3 are mounted on one circuit board 5 in the COB device as shown in FIG. 5, the circuit board 5 is warped in the longitudinal direction. This is significant in a silicon substrate with a thickness of 625 μm, length of 100 mm, and width of 7 mm having 8 to 9 thin bare IC chips with a thickness of 175 μm, length of 10 mm, and width of 2.5 mm mounted thereon, where it is confirmed that the substrate is warped by 40 to 50 μm.
A fourth problem is that in a method for pressing the IC 3 with an elastic sheet interposed therebetween, disclosed in Japanese Patent Laid-Open No. 2000-68633, as shown in FIG. 6, when the back face of the IC chip 3 is inclined relative to the pressing direction, nonuniform distribution in the pressure is produced, resulting in nonuniformity of crush amounts of the bumps 4 and of a connection state.
Furthermore, as shown in FIG. 7, there is also a problem that because the elastic sheet 9 pressurizes the back surface of the IC chip 3 with a uniformly distributed force, portions, particularly where the bumps 4 do not exist in the bump arrangement, cave in, resulting in a warp of the IC chip 3. The warp of the IC chip 3 is significant especially in the case of a thin IC. When the warped IC is bonded and mounted, an ACF resin changes in quality and is softened while the warped IC chip is to get back, after a heating process performed thereafter or a reliability test. Then, as shown in FIG. 8, a largely crushed bump 4a is separated from the substrate 5, resulting in defective bonding.
Moreover, in the mounting method using the elastic sheet, it is difficult for the elastic sheet to follow the height differences and different force may be applied of the IC chips for adjacent chips. If there are differences in height, the elastic sheet follows the higher IC chips, so that the lower IC chips are not abutted by the elastic sheet and not subjected to a force.